PFM (Posterior) Crown Restorations : restorative Dentistry Lecture note

 
PFM (Posterior) Crown Restorations
Indications

·         Teeth are short, tapered, round or lack well developed cingulum
·         Patient with occlusal habit that place heavy forces on the restoration
·         As retainer for RPD
·         To correct minor, malalignment and mal occlusion
·         Severly destructive tooth
·         Endodontically treated tooth and tooth with post and core
·         Correction if abutment tooth for RPD
·         Long span bridges

Contra-indications

·         Young patient due to large pulp chamber and incomplete crown eruption.
·         Patient with active caries and untreated periodontal disease.
·         Intact abutment teeth in short span ridges.
·         When the teeth can be satisfactory restored with a more conservative restoration.

Advantages

·         Glazed porcelain is the most esthetically durable and biocompatible restorative material for teeth (resist wear and plaque adhesion)
·         The metal substructure provide support to the porcelain and hence less liable to fracture than the all porcelain crown
·         The metal substructure allowed the fabrication of fixed partial denture
·         The natural appearance can be closely matched by characterization with internal and/or external staining techniques
·         The preparation of the PFM is much easier than that of the partial coverage restoration

Disadvantages

·         Need a less conservative preparation
·         Less esthetic qualities than the all porcelain crown
·         Needs special equipment and expensive armentarium
·         Fracture liability of the porcelain veneering
·         facial gingival finish line should be carried subgingivally and consequently more potential for gingival involvement and periodontitis
·         Difficult of making an accurate shade reproduction due to metal

Steps

·         Occlusal reduction
·         Axial reduction
·         marginal placement
·         Seating grooves
·         Finishing

Occlusal reduction (posterior) or incisal reduction (anterior)

·         We do depth orientation grooves for even thickness reduction then remove tooth structure in between
·         We use 170 or 171 tapered fissure bur and round-end tapered diamond bur
In lower teeth:
·         2mm reduction in the bucaal cusps (functional cusps)
·         1mm reduction in lingual cusps

In upper teeth:
·         1.5mm reduction for both buccal and lingual cusps (functional cusps)
Functional cusp bevel
·         Is done on both upper teeth (on lingual cusps) and lower teeth (on buccal cusps) with round end tapered diamond bur and bur no.171 the depth is 2mm
·         Angle between functional cusp bevel and functional cusp slope should be 45 degree
·         -To follow the morphology of the tooth
·         -Stability and support
·         -Prevent fracture
·         -Prevent PDL injury
·         -Lateral torque prevention

Axial reduction

·         Buccal and lingual walls reduction then proximal walls reduction
·         We use flat –end tapered diamond bur (shoulder finish line) hold it parallel to the long axis of the tooth otherwise undercut or over tapering will occur
·         For lingual reduction we use turbido bur. or we use the round end tapered diamond bur (chamfer finish line)
·         To open the contact we use short thin tapered diamond bur

Others

We may use retentive grooves if the tooth is too short so we need extra means of retention
Wings at the end of the shoulder are done for:
1.       prevent horizontal movement
2.       preserve tooth structure.
3.       Marginal placement

 Click here to Download Compare and Contrast among FullCrowns, PFM anterior, PFM posterior, Three quarter anterior and Three QuarterPosterior

Key Words : Porcelain fused to metal Crowns, Ceramic fused to metal crowns
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